A combustion engine comprises combustion chambers (1-4) with reciprocating pistons (5), intake ports (6) and exhaust ports (7). Overflow ports (11,12) are provided between adjacent combustion chambers to provide an overflow channel (15,16) that closes during a high load mode of operation of said engine and opens during a partial load mode of operation. The overflow ports (11,12) straddle a path of shortest distance between adjacent combustion chambers and said overflow channel (15) extends at least substantially along said path of shortest distance. In a further aspect of the invention, exhaust ports (1b+2a, 3b+4a) of adjacent combustion chambers are joined into a common exhaust channel (P2,P4) that communicates with an exhaust header (20) of the engine through valve means (V1,V2) that open during the high load mode of operation of said engine and close during a partial load mode of operation.